Self-seal tire and its producing method

ABSTRACT

A self-sealing tire according to the present invention includes a pneumatic tire and a puncture sealing layer disposed over a whole tire inner wall surface from one shoulder portion to the other shoulder portion of the pneumatic tire. The puncture sealing layer comprises an annular sponge layer which is impregnated with an adhesive so as to be exposed on a radially outer surface side thereof, and a bonding layer of an adhesive which is provided on a radially outer surface of the sponge layer and is adhered to the tire inner wall surface. The sponge layer is provided in a compressed state. The puncture sealing layer is press attached to the tire inner wall surface by a recovery force of the compressed sponge layer.

TECHNICAL FIELD

[0001] The present invention relates to a self-sealing tire andmanufacturing method of the same, having a self-sealing function ofsealing a puncture to prevent air from leaking outside the tire when itblows out by a nail or the like penetrating its crown portion duringtraveling of a vehicle.

TECHNICAL BACKGROUND

[0002] With the diffusion of highways, the demand for tires having aself-sealing function has been increasing day by day, and variousself-sealing tires with a puncture sealing layer have been proposedheretofore. Conventionally, there has been proposed, for example, aself-sealing tire in which a high viscosity adhesive is applied as asealing material to the inner surface of the tire crown portion to forma puncture sealing layer. The tire of this type, however, involves aproblem that, when the tire turns at a high seed, the adhesive can notseal a puncture in the vicinity of the shoulder portions, because itflows to the center side of the crown portion by a centrifugal force.

[0003] Japanese Patent Laid-open No. 1979-6206 discloses a self-sealingtire in which a sponge layer impregnated with an adhesive similar to theaforementioned is affixed to the inner surface of the tire crown portionto form a puncture sealing layer. The puncture sealing layer of thistire is formed such that, after attaching a sponge layer to the tireinner surface, an adhesive is impregnated into the sponge layer from onesurface thereof by means of press rollers with the tire being rotated.

[0004] Consequently, in order to fully impregnate the sponge layer withthe adhesive so that it reaches the tire inner surface, it is necessaryto turn the tire at a low speed and to provide a high pressing force. Asa result, the working time for impregnation of the adhesive increases,thereby deteriorating workability.

[0005] Since the adhesive is exposed on the surface of the sponge layer,dust and foreign object stick thereto during transporting, stocking andthe like, thereby deteriorating its appearance, and the adhesive adheresto a hand of the operator and equipment during handling of the tire,thereby making it bad to handle the tire.

DISCLOSURE OF THE INVENTION

[0006] It is an object of the present invention to provide aself-sealing tire and a manufacturing method thereof, which is excellentin handling, prevents an adhesive from moving during traveling of avehicle, improves workability in forming a puncture sealing layer, anddoes not deteriorate its appearance.

[0007] In order to achieve the aforementioned object, a self-sealingtire according to the present invention includes a pneumatic tire and apuncture sealing layer disposed over a whole tire inner wall surfacefrom one shoulder portion to the other shoulder portion of the pneumatictire, the puncture sealing layer comprising an annular sponge layerwhich is impregnated with an adhesive so as to be exposed on a radiallyouter surface side thereof, and a bonding layer of an adhesive which isprovided on a radially outer surface of the sponge layer and is adheredto the tire inner wall surface, the sponge layer being provided in acompressed state, and the puncture sealing layer being press attached tothe tire inner wall surface by a recovery force of the compressed spongelayer.

[0008] A self-sealing tire manufacturing method according to the presentinvention, in which the self-sealing tire includes a pneumatic tire anda puncture sealing layer disposed over a whole tire inner wall surfacefrom one shoulder portion to the other shoulder portion of the pneumatictire, comprises the steps of: impregnating an annular sponge layer withan adhesive from a radially outer surface side thereof to form anadhesive impregnated sponge layer so that the adhesive is exposed on theradially outer surface side; forming a bonding layer made of an adhesiveon a radially outer surface of the adhesive impregnated sponge layer toform a puncture sealing layer; and press attaching the puncture sealinglayer onto the tire inner wall surface via the bonding layer by arecovery force of the sponge layer which has compressed.

[0009] Another self-sealing tire manufacturing method according to thepresent invention, in which the self-sealing tire includes a pneumatictire and a puncture sealing layer disposed over a whole tire inner wallsurface from one shoulder portion to the other shoulder portion of thepneumatic tire, comprises the steps of: impregnating an annular spongelayer with an adhesive from a radially outer surface side thereof toform an adhesive impregnated sponge layer so that the adhesive isexposed on the radially outer surface side, and to simultaneously form abonding layer made of the adhesive on a radially outer surface of theadhesive impregnated sponge layer integrally therewith, thereby forminga puncture sealing layer; and press attaching the puncture sealing layeronto the tire inner wall surface via the bonding layer by a recoveryforce of the sponge layer which has compressed.

[0010] According to the present invention described above, since theadhesive for sealing a puncture is impregnated into the sponge layer,flow of the adhesive by a centrifugal force to the center side of thecrown portion can be prevented during traveling of a vehicle. Apuncture, therefore, can be sealed in the vicinity of the shoulders.

[0011] Since the adhesive is impregnated so as to be exposed on theradially outer surface side of the sponge layer and not to be exposed onthe radially inner surface side thereof, sticking of dust and foreignobject to the radially inner surface of the sponge layer can be avoidedduring transporting, stocking and the like. As a result, thedeterioration of appearance can be avoided. Also, the adhesive does notstick to a hand of the operator and equipment during handling of thetire, thereby facilitating the tire handling.

[0012] Since the adhesive is previously impregnated into the spongelayer and the puncture sealing layer is press attached to the tire innerwall surface by a recovery force of the compressed sponge layer, thepuncture sealing layer can be easily and tightly bonded to the tireinner wall surface without press attachment operation by application ofa high compression force from the outside by means of press rollers orthe like. Accordingly, an improved workability can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a cross-sectional view of one-half of a self-sealingtire according to the present invention, taken along the tire meridian;

[0014]FIG. 2 is an enlarged section of the part indicated by the arrow Ain FIG. 1;

[0015]FIG. 3 is a perspective view of an annular sponge layer prior toattachment of a puncture sealing layer;

[0016]FIG. 4 is a schematic view showing a step of forming a puncturesealing layer in a self-sealing tire manufacturing method according tothe present invention;

[0017]FIG. 5 is an enlarged section showing a main part of anotherpuncture sealing layer to be attached to a self-sealing tire accordingto the present invention; and

[0018]FIG. 6 is a schematic view showing a step of forming the puncturesealing layer of FIG. 5.

BEST MODES FOR CARRYING OUT THE INVENTION

[0019] The embodiments of the present invention will be described belowwith reference to the drawings. In respective drawings, the samecomponents are assigned the same symbols, and duplicated descriptionwill be omitted.

[0020] Referring to FIG. 1, the self-sealing tire of the presentinvention comprises a pneumatic tire 1 and a puncture sealing layer 2attached to an entire tire inner wall surface 1X from one shoulderportion S to the other shoulder portion S of the pneumatic tire 1.

[0021] The pneumatic tire 1 has a crown portion 11, right and leftsidewall portions 12, and right and left bead portions 13. Providedbetween the right and left bead portions 13 are a carcass layer 14, bothend portions 14 a of which are turned axially outwardly around a beadcore 15 embedded in each of the bead portions 13. A plurality of beltlayers 16 are disposed radially outwardly of the carcass layer 14 in thecrown portion 11. The tire centerline is designated by CL.

[0022] The puncture sealing layer 2 is formed in an annular shape and,as shown in FIG. 2, comprises a sponge layer 3 impregnated with a highviscosity adhesive X as sealing material so that it is not exposed onthe radially inner surface side of the sponge layer but is exposed onthe radially outer surface side thereof, and an bonding layer 4, made ofan adhesive, which is provide on the radially outer surface 3 a of thesponge layer 3 and bonds to the tire inner wall surface 1X.

[0023] The adhesive used for the bonding layer 4 is composed of the samehigh viscosity adherent composition as the adhesive X impregnated intothe sponge layer 3, and the bonding layer 4 is integrally formed on theradially outer surface 3 a of the sponge layer 3.

[0024] The sponge layer 3, which is annularly shaped and is elastic, isarranged such that the peripheral length (outer circumference length) Lsof the radially outer surface of the sponge layer 3 prior to attachmentof the puncture sealing layer 2 to the pneumatic tire 1 (see FIG. 3) islonger than the tire inner circumference length Lt of the tire innerwall surface 1X between the shoulder portions S to which the puncturesealing layer 2 is attached, the sponge layer 3 being attached in aradially inwardly compressed state, thereby press attaching the puncturesealing layer 2 to the tire inner wall surface 1X by a restoring forceof the sponge layer 3 acting radially outwards.

[0025] The self-sealing tire having the aforementioned puncture sealinglayer 2 is manufactured as follows.

[0026] As shown in FIG. 4, adhesive X discharged from a discharge port 6of an adhesive extruder is consecutively press attached by means of apair of rollers 7, 7 onto the radially outer surface 3 a of the annularpreformed sponge layer 3 rotated by rollers 5, 5 so as to round theradially outer surface 3 a. The adhesive X is impregnated into thesponge layer 3 from the radially outer surface 3 a during this process,thereby forming an adhesive impregnated sponge layer 3′ which has theadhesive X not exposed on the radially inner surface side, but exposedon the radially outer surface side thereof, and simultaneously formingthe bonding layer 4 made of the adhesive X on the radially outer surface3′a of the adhesive impregnated sponge layer 3′(the radially outersurface 3 a of the sponge layer 3) to form the puncture sealing layer 2.

[0027] Next, the obtained puncture sealing layer 2 is compressed andintroduced inside the pneumatic tire 1, and is then decompressed. Thepuncture sealing layer 2 is pressed to the tire inner wall surface 1Xwith the bonding layer 4 being stuck closely thereto by a restoringforce of the sponge layer 3 trying to restitute to its original size,whereby the puncture sealing layer 2 is press attached and tightlybonded to the tire inner wall surface 1X without press attaching thepuncture sealing layer 2 thereto by applying a high compression forcefrom the outside.

[0028] According to the self-sealing tire of the present inventionmentioned above, flow of the adhesive X by a centrifugal force can beprevented during traveling of a vehicle, since the adhesive X used forthe puncture sealing layer 2 is impregnated into the sponge layer 3.Consequently, a good puncture sealing ability can be achieved even inthe vicinity of the shoulder portions S because of no flow of theadhesive X to the center side of the crown portion 11.

[0029] Since the adhesive X is impregnated so as to be exposed on theradially outer surface side of the sponge layer 3 and not to be exposedon the radially inner surface side thereof, dust and foreign object areprevented from sticking during transporting, stocking and the like, thusnot deteriorating the appearance. Also, the adhesive does not stick to ahand of the operator and equipment during handling of the tire, therebyimproving the handling performance.

[0030] Since the adhesive X is previously impregnated into the spongelayer 3 and the puncture sealing layer 2 is press attached to the tireinner wall surface X by a recovery force of the compressed sponge layer3, the puncture sealing layer 2 can be easily and tightly bonded to thetire inner wall surface 1X without press attachment operation byapplication of a high compression force from the outside by means ofpress rollers or the like. Accordingly, a good workability can beobtained.

[0031]FIG. 5 shows another puncture sealing layer used for theself-sealing tire of the present invention. This puncture sealing layer2 has a bonding layer 4 which is composed of a high viscosity adhesive Ymade of an adherent composition different from the adhesive X and whichhas a high viscosity sealing capacity.

[0032] Such a puncture sealing layer 2 is formed as follows. As shown inFIG. 6, adhesive X discharged from a discharge port 6 of a firstadhesive extruder is consecutively press attached by means of a pair ofrollers 7, 7 onto an radially outer surface 3 a of an annular spongelayer 3 rotated by rollers 5, 5. The adhesive X is impregnated into thesponge layer 3 from the radially outer surface 3 a, thereby sequentiallyforming an adhesive impregnated sponge layer 3′ such that the adhesive Xis not exposed to the radially inner surface side but exposed to theradially outer surface side thereof.

[0033] Next, adhesive Y discharged from a discharge port 8 of a secondadhesive extruder is consecutively press attached onto the radiallyouter surface 3 a′ of the adhesive impregnated sponge layer 3′ by meansof a pair of rotating rollers 9, 9. The bonding layer 4 is sequentiallyformed on the radially outer surface 3 a′. The puncture sealing layer 2is formed by applying the adhesives X, Y so that they round the spongelayer 3.

[0034] The puncture sealing layer 2 formed as mentioned above is bondedto the tire inner wall surface 1X in a similar manner to that describedabove.

[0035] Effects similar to the aforementioned can be obtained even thoughthe bonding layer 4 of the puncture sealing layer 2 is composed of theadhesive Y different from the adhesive X as illustrated above.

[0036] The puncture sealing layer 2 shown in FIG. 5 may be formed byformation of the bonding layer 4 on the tire inner wall surface 1Xbeforehand, and by adhesion of an adhesive impregnated sponge layer 3′tothe bonding layer 4. It should be appreciated that the bonding layer 4formed on the tire inner wall surface may be formed using the adhesiveX.

[0037] In the present invention, it is preferable that a relationshipbetween the outer circumference length Ls of the sponge layer 3 and thetire inner circumference length Lt of the tire inner wall surface 1X beset to 1.1 Lt≦Ls≦1.3 Lt on the tire centerline CL.

[0038] If the outer circumference length Ls of the sponge layer 3 isless than 1.1 Lt, it is difficult to firmly adhere the puncture sealinglayer 2 to the tire inner wall surface 1X by the recovery force of thesponge layer 3, deteriorating the puncture sealing ability. If it islarger than 1.3Lt, the circumference length of the sponge layer 3 is solong that the sponge layer 3 is partially bent radially inward, creatingan insufficiently adhered part, and when a puncture happens in the partby a nail or the like, the part peels off from the tire inner wall face1X, causing the deterioration of a puncture sealing ability.

[0039] The thickness of the sponge layer 3 prior to attachment of thepuncture sealing layer and the impregnation depth of the adhesive X withwhich the sponge layer 3 is impregnated may conveniently be selectedaccording to the size of a pneumatic tire to be attached. From theviewpoint of prevention of excessive increase in tire weight, and of agood sealing ability, it is preferable to set the thickness of thesponge layer 3 to be 5 to 15 mm, and the impregnation depth of theadhesive X to be 4 to 12 mm. The impregnation amount of adhesive X ispreferably 0.1 to 0.5 g/cm^(3.)

[0040] Material composing the aforementioned sponge layer 3 maypreferably include a rubber having a continuous bubble into which theadhesive X can be impregnated, and a foam resin formed of polyurethane,nylon or the like. It is desirable to employ a continuous bubble typeether system polyurethane foam.

[0041] As adherent composition used for the adhesive X, an elastomerblended with an amorphous polyolefin, paraffin oil, or tackifier such aspetroleum resin or low molecular weigh oligomer which is, for example,liquid polybutene may preferably be used. The elastomer may preferablybe, for instance, at least one type of elastomer selected from a groupof butyl rubber, polyisobutylene, natural rubber, isoprene rubber andpolybutene in order to enhance the air impermeability of the tire in anormal use state with no puncture.

[0042] Also, in case of forming the bonding layer 4 with the adhesive X,the aforementioned elastomer blended with tackifier such as petroleumresin or low molecular weigh oligomer which is, for example, liquidpolybutene may be used as the adhesive.

[0043] The adherent composition used for the adhesive Y may be one asused in the case of forming the bonding layer 4 with the adhesive X.

[0044] An appropriate crosslinking agent or vulcanizing agent may beblended with the adhesive X, Y in order to provide heat resistance forthe adhesive. For instance, in the case of using butyl rubber orpolybutene, p-quinonedioxime or the like may be blended.

[0045] It is preferable that the adhesives X, Y be made of an adherentcomposition not containing solvent, and that the viscosity thereof isadjusted to be about 10000Pa·s.

EXAMPLE 1

[0046] Tires of the present invention and of the comparison weremanufactured, having a tire size of 205/65R15 and a tire constructionshown in FIG. 1, the tire of the present invention having a puncturesealing layer which was press attached to the tire inner wall surface bya restoring force of the sponge layer with its outer circumferencelength Ls longer than the tire inner circumference length Lt of the tireinner wall surface, and the comparative tire having a puncture sealinglayer which was attached to the tire inner wall surface with the spongelayer thereof having an outer circumference length Ls equal to the tireinner circumference length Lt of the tire inner wall surface.

[0047] The outer circumference length Ls of the sponge layer of thepresent invention at a position corresponding to that of the tirecenterline prior to attachment was 1.2 times longer than the tire innercircumference length Lt of the tire inner wall surface on the tirecenterline.

[0048] Each sponge layer used for the present invention and comparativetires was composed of a continuous bubble type ether system polyurethanefoam, and a composition having butyl rubber and liquid polybutene asmain components was used for the adhesive.

[0049] The thickness of each sponge layer used for the present inventionand comparative tires was 10 mm and the adhesive impregnation depth was8 mm.

[0050] The test tires were each attached to a rim having a size of15×6JJ, with the tire air pressure being set to 200 kPa, and a puncturesealing ability evaluation test was performed under the followingmeasurement method, thereby obtaining the results shown in Table 1.

[0051] Puncture Sealing Ability

[0052] A nail was stuck through the crown portion and puncture sealinglayer of each test tire, and then extracted, and each test tire was leftas it was for 24 hours. Thereafter the air pressure of each test tirewas measured, and each result was evaluated in two levels of ∘and X.∘means that there was no air leak, and X means that air leak took place.TABLE 1 Present invention tire Comparative tire Puncture sealing ability◯ X

[0053] A can be seen from Table 1, the present invention tire having apuncture sealing layer which was press attached to the tire inner wallsurface by a restoring force of the sponge layer can demonstrate a goodpuncture sealing ability.

EXAMPLE 2

[0054] Test tires 1 to 5 were respectively manufactured, having the sametire size and construction as in Example 1, the test tires each having apuncture sealing layer disposed on the tire inner wall surface, theouter circumference length Ls of each sponge layer at a positioncorresponding to that of the tire centerline prior to attachment withrespect to the tire inner circumference length Lt of the tire inner wallsurface at a position of the tire centerline being changed as shown inTable 2.

[0055] Each sponge layer used for the test tires was composed of acontinuous bubble type ether system polyurethane foam, and a compositionhaving butyl rubber and liquid polybutene as main components was usedfor the adhesive. The thicknesses t of the sponge layers and theadhesive impregnation depths h were shown in Table 2, respectively.

[0056] A puncture sealing ability evaluation test of each test tire wascarried out as in Example 1, thereby obtaining the results shown inTable 2. TABLE 2 Test tire 1 Test tire 2 Test tire 3 Test tire 4 Testtire 5 Outer 1.05 Lt 1.10 Lt 1.20 Lt 1.30 Lt 1.40 Lt circumfer- encelength Ls Thickness t 10 10 10 10 10 (mm) Depth h  8  6  8  8  8 (mm)Puncture X ◯ ◯ ◯ X sealing ability

[0057] From Table 2, it is appreciated that the outer circumferencelength Ls of the sponge layer may be 1.1 to 1.3 times longer than thetire inner circumference length Lt of the tire inner wall surface.

[0058] As mentioned above, the present invention can prevent theadhesive from flowing to the center side of the crown portion by acentrifugal force generated during traveling of a vehicle, since theadhesive used for the puncture sealing layer is impregnated into thesponge layer.

[0059] Since the adhesive is not exposed on the radially inner surfaceside of the sponge layer, but is exposed on the radially outer surfaceside thereof, the deterioration of appearance due to sticking of dustand foreign object during transporting, stocking and the like does notoccur, and a good handling performance can be obtained.

[0060] Since the adhesive is previously impregnated into the spongelayer and the puncture sealing layer is press attached to the tire innerwall surface by a recovery force of the compressed sponge layer, thepuncture sealing layer can be easily and tightly bonded to the tireinner wall surface without press attachment operation by application ofa high compression force from the outside by means of press rollers orthe like, thereby attaining a good workability.

INDUSTRIAL APPLICABILITY

[0061] The present invention having the aforementioned excellent effectscan be used very effectively for a self-sealing tire having aself-sealing function of sealing a puncture to prevent air from leakingoutside the tire when it blows out by a nail or others penetrating itscrown portion during traveling of a vehicle.

1. A self-sealing tire having a pneumatic tire and a puncture sealinglayer disposed over a whole tire inner wall surface from one shoulderportion to the other shoulder portion of the pneumatic tire, wherein thepuncture sealing layer comprises an annular sponge layer which isimpregnated with an adhesive so as to be exposed on a radially outersurface side thereof, and a bonding layer of an adhesive which isprovided on a radially outer surface of the sponge layer and is adheredto the tire inner wall surface, the sponge layer being provided in acompressed state, the puncture sealing layer being press attached to thetire inner wall surface by a recovery force of the compressed spongelayer.
 2. The self-sealing tire of claim 1, wherein a relationshipbetween an outer circumference length Ls of the sponge layer at aposition corresponding to that of the tire centerline of the pneumatictire prior to attachment of the puncture sealing layer and a tire innercircumference length Lt of the tire inner wall surface at a position ofthe tire centerline is set to 1.1 Lt≦Ls≦1.3 Lt.
 3. The self-sealing tireof claim 1, wherein the bonding layer is made of the same adherentcomposition as the adhesive with which the sponge layer is impregnated.4. The self-sealing tire of claim 3, wherein said bonding layer isintegrally formed on the radially outer surface of the sponge layer. 5.The self-sealing tire of claim 1, wherein the thickness of the spongelayer prior to attachment of the puncture sealing layer is 5 to 15 mm.6. The self-sealing tire of claim 5, wherein the impregnation depth ofthe adhesive which is impregnated into the sponge layer prior toattachment of the puncture sealing layer is 4 to 12 mm.
 7. Theself-sealing tire of any one of claims 1 to 6, wherein the adhesives aremade of an adherent composition not containing solvent.
 8. A method ofmanufacturing a self-sealing tire having a pneumatic tire and a puncturesealing layer disposed over a whole tire inner wall surface from oneshoulder portion to the other shoulder portion of the pneumatic tire,comprising the steps of: impregnating an annular sponge layer with anadhesive from a radially outer surface side thereof to form an adhesiveimpregnated sponge layer so that the adhesive is exposed on the radiallyouter surface side; forming a bonding layer made of an adhesive on aradially outer surface of the adhesive impregnated sponge layer to forma puncture sealing layer; and press attaching the puncture sealing layeronto the tire inner wall surface via the bonding layer by a recoveryforce of the sponge layer which has compressed.
 9. The self-sealing tiremanufacturing method of claim 8, wherein a relationship between an outercircumference length Ls of the sponge layer at a position correspondingto that of the tire centerline of the pneumatic tire prior to attachmentof the puncture sealing layer and a tire inner circumference length Ltof the tire inner wall surface at a position of the tire centerline isset to 1.1 Lt≦Ls≦1.3 Lt.
 10. The self-sealing tire manufacturing methodof claim 8 or 9, wherein the adhesives are made of an adherentcomposition not containing solvent.
 11. A method of manufacturing aself-sealing tire having a pneumatic tire and a puncture sealing layerdisposed over a whole tire inner wall surface from one shoulder portionto the other shoulder portion of the pneumatic tire, comprising thesteps of: impregnating an annular sponge layer with an adhesive from aradially outer surface side thereof to form an adhesive impregnatedsponge layer so that the adhesive is exposed on the radially outersurface side, and to simultaneously form a bonding layer made of theadhesive on a radially outer surface of the adhesive impregnated spongelayer integrally therewith, thereby forming a puncture sealing layer;and press attaching the puncture sealing layer onto the tire inner wallsurface via the bonding layer by a recovery force of the sponge layerwhich has compressed.
 12. The self-sealing tire manufacturing method ofclaim 11, wherein a relationship between an outer circumference lengthLs of the sponge layer at a position corresponding to that of the tirecenterline of the pneumatic tire prior to attachment of the puncturesealing layer and a tire inner circumference length Lt of the tire innerwall surface at a position of the tire centerline is set to 1.1Lt≦Ls≦1.3 Lt.
 13. The self-sealing tire manufacturing method of claim 11or 12, wherein the adhesive is made of an adherent composition notcontaining solvent.
 14. The self-sealing tire of claim 2, wherein thebonding layer is made of the same adherent composition as the adhesivewith which the sponge layer is impregnated.
 15. The self-sealing tire ofclaim 2, wherein the thickness of the sponge layer prior to attachmentof the puncture sealing layer is 5 to 15 mm.
 16. The self-sealing tireof claim 3, wherein the thickness of the sponge layer prior toattachment of the puncture sealing layer is 5 to 15 mm.
 17. Theself-sealing tire of claim 4, wherein the thickness of the sponge layerprior to attachment of the puncture sealing layer is 5 to 15 mm.